Honeywell T8400C1099 Electronic Thermostats Installation Guide

: June 17, 2024
: Honeywell

Table of Contents

Honeywell T8400C1099 Electronic Thermostats
Product Information
Product Usage Instructions
Frequently Asked Questions
INSTALLATION INSTRUCTIONS
Mounting Wallplate to Wall
Wiring
OPERATION
CHECKOUT
References
Read User Manual Online (PDF format)
Download This Manual (PDF format)

Honeywell T8400C1099 Electronic Thermostats

Product Information

Specifications:

Model: T8400C1099, T8401C1031
Power Source: T8400 – Powered through heating/cooling system controls, T8401 – Requires a common wire
Batteries: Not required
Memory: Setpoints held permanently by non-volatile memory
Included Components: Thermostat, Wallplate, Owner’s Guide

Product Usage Instructions

Installation:

Read these instructions carefully before installation.
Check the ratings on the product to ensure suitability for your application.
Only a trained, experienced service technician should perform the installation.
Disconnect the power supply before installation to prevent electrical shock or equipment damage.
After installation is complete, check the product operation as provided in the instructions.

Location:

Install the thermostat approximately 5 ft (1.5m) above the floor in an area with good air circulation and average temperature. Avoid installing the thermostat in places affected by:

Drafts or dead spots behind doors and in corners.
Hot or cold air from ducts.
Radiant heat from the sun or appliances.
Concealed pipes and chimneys.
Unheated (uncooled) areas such as an outside wall behind the thermostat.

Mounting Wallplate to Wall:

Note: Leveling is only for appearance, the thermostat functions normally even when not level.

Place the wallplate at the desired location on the wall.
Pull the thermostat wire through the entrance hole on the wallplate.
Fasten the wallplate to the wall using the provided anchors and screws.
After wiring the wallplate, plug the hole to prevent drafts from affecting the thermostat.

Wiring:

Use an 18-gauge maximum wire to wire the T8400 and T8401 Thermostats. All wiring must comply with local electrical codes and ordinances. Disconnect the power supply to prevent electrical shock or equipment damage.

Note: Restrict all wiring to the shaded area for proper mounting of the thermostat.

The shape of the terminals permits insertion of straight or wraparound wiring connections; either method is acceptable.

Frequently Asked Questions

Q: Do I need batteries for the T8400 and T8401 Thermostats?

A: No, batteries are not required for these thermostats as setpoints are held permanently by non-volatile memory.

Q: Can I install the thermostat in any location?

A: It is recommended to install the thermostat approximately 5 ft (1.5m) above the floor in an area with good air circulation and average temperature. Avoid installing it in places affected by drafts, hot or cold air from ducts, radiant heat, concealed pipes, chimneys, or unheated/uncooled areas such as an outside wall behind the thermostat.

Q: What wire gauge should I use for wiring the thermostat?

A: Use an 18-gauge maximum wire for wiring the T8400 and T8401 Thermostats.

Q: Can I level the thermostat for appearance?

A: Yes, you can level the thermostat for appearance, but note that it functions normally even when not level.

INSTALLATION INSTRUCTIONS

The T8400 and T8401 Thermostats provide single-stage, non-programmable temperature control for 24V heating-cooling systems with manual changeover from heat to cool. Heating cycle rate is selectable at 1, 3, 4, 5, 6, 9, or 12 cph. Cooling cycle rate is fixed at 3 cph. Temperature indication can be set for °F or °C. The T8400 Thermostat is powered through the heating/cooling system controls. The T8401 requires a common wire for operation. Batteries are not required because setpoints are held permanently by non-volatile memory. The T8400 and T8401 models include a thermostat, wallplate (for wiring and mounting thermostat) and owner’s guide.

MERCURY NOTICE

If this control is replacing a control that contains mercury in a sealed tube, do not place your old control in the trash. Contact your local waste management authority for instructions regarding recycling and the proper disposal of an old control containing mercury in a sealed tube.

INSTALLATION

When Installing this Product…

Read these instructions carefully. Failure to follow them could damage the product or cause a hazard-ous condition.
Check the ratings given in the instructions and on the product to make sure the product is suitable for your application.
The installer must be a trained, experienced service technician.
After installation is complete, check out product operation as provided in these instructions.

CAUTION

Electrical Shock or Equipment Damage Hazard. Can shock individuals or short equipment circuitry. Disconnect power supply before installation.

Location

Install the thermostat about 5 ft (1.5m) above the floor in an area with good air circulation at average temperature. See Fig. 1. Do not install the thermostat where it can be affected by:

drafts or dead spots behind doors and in corners.
hot or cold air from ducts.
radiant heat from the sun or appliances.
concealed pipes and chimneys.
unheated (uncooled) areas such as an outside wall behind the thermostat.

This thermostat is a precision instrument and was carefully adjusted at the factory. Handle it carefully.

Mounting Wallplate to Wall

IMPORTANT

Level only for appearance. The thermostat functions normally even when not level.

Mount the T8400 and T8401 with the screws provided (see Fig. 2) as follows:

Place the wallplate at the desired location on the wall.
Pull the thermostat wire through the entrance hole on the wallplate.
Fasten the wallplate to the wall using the anchors and screws provided.
After wiring the wallplate, plug the hole to prevent drafts from affecting the thermostat; see the Wiring section.

Setting Fan Operation (Fuel) Switch

The fan operation (fuel) switch is preset at the factory in the F position. See Fig. 3. This is the correct setting for most systems. If this system is an electric heat system, set the switch to the E position. The E setting allows the fan to turn on immediately with the heating or cooling equipment in a system where the G terminal is connected.

Wiring

IMPORTANT

Use an 18-gauge maximum wire to wire the T8400 and T8401 Thermostats.

All wiring must comply with local electrical codes and ordinances. Disconnect the power supply to prevent electrical shock or equipment damage.
NOTE: To ensure proper mounting of thermostat, restrict all wiring to the shaded area. See Fig. 4 The shape of the terminals permits insertion of straight or wraparound wiring connections; either method is acceptable. See Fig. 5. Refer to Fig. 6 through 10 for typical wiring hookups.

Mounting Thermostat to Wallplate

Engage the tabs at the top of the thermostat and wallplate.
Swing down the thermostat and press the lower edge of the thermostat onto the wallplate to latch. See Fig. 11.

OPERATION

Setting FAN and SYSTEM Switches

Fan and system settings are controlled manually by using the switches located at the bottom of the thermostat case. See Fig. 12.

FAN Switch

Fan switch settings are:

On: The fan runs continuously. Use for improved air circulation and air quality.
Auto: Normal setting for most homes. In cooling, the fan starts and stops with the cooling equipment. In heating, the fan is controlled directly by the heating equipment and may start a few minutes after the heating equipment turns on (on most systems). When using an electric heat thermostat, the fan starts and stops with the heating equipment. Slide the FAN switch in the bottom left corner of the thermostat to select the desired fan setting.

SYSTEM Switch

System switch settings control thermostat operation as follows:

Cool: The thermostat controls the cooling system.
Off: Both heating and cooling are off.
Heat: The thermostat controls the heating system.

Slide the SYSTEM switch in the bottom right corner of the thermostat to select the desired system setting.

Set Temperature Setpoint

NOTE: Temperature setpoint range is 40° to 99°F (5° to 37°C).
The temperature setpoint and the room temperature are shown separately on the digital display. The ▼ indicator points to Set when the temperature setpoint is displayed and to Room when the room temperature is displayed.

To set temperature setpoint:

Select Heat or Cool by sliding the SYSTEM switch in the lower right corner of the thermostat to the desired mode. See Fig. 12.
To display the temperature setpoint on the digital display, press either the ▲ or ▼ key once. The tem-perature setpoint is displayed for approximately five seconds as the indicator points to Set and flashes.
To increase the temperature setpoint press the ▲ key. Each press changes the setpoint one degree; press and hold to change the setpoint several degrees.
To decrease the temperature setpoint press the ▼ key. Each press changes the setpoint one degree; press and hold to change the setpoint several degrees.

Setting °F/°C Indication and Heat Cycle Rate

NOTE: To save changes to the °F/°C indication, the heat cycle rate and the C3/C1 temperature control, tap the ▲ key until exiting the installer setup.

To set the °F/°C indication, the heat cycle rate and the C3/C1 temperature control:

If the temperature is displayed in °F, set the temperature setpoint to 52°F. If the room temperature is displayed in °C, set the temperature setpoint to 11°C.
OPTIONAL SYSTEM CHECKOUT

When in steps 2 and 3 only, the ▼ key can be used to turn heat or cool outputs on and off. Change the SYSTEM switch setting to test heat or cool outputs. No action takes place If the system switch is in the Off position.
Examples: System setting at HEAT: If heat is on, the ▼ key turns it off; if heat is off, ▼ turns it on.
System setting at COOL: If cool is on, ▼ key turns it off; if cool is off, ▼ turns it on.

Press the ▲▼ keys simultaneously for more than one second to light all segments on display and to enter installer setup mode. When the keys are released, a two-digit software revision code is displayed.
NOTE: In installer setup mode only, each press of the key▲ momentarily displays 01. Each press of the ▼ key momentarily displays 02. When the keys are released, these two-digit codes are no longer displayed.
Press the ▲ key. Factory information is displayed. A typical example is shown, but information dis-played varies by model. This information is for fac-tory use only.
Press the ▲ key again to display °C or °F.
Press the ▼ key to change the °C or °F indication.
Press the ▲ key to display the heat cycle rate of 1, 3, 4, 5, 6, 9, or 12. To change the heat cycle rate, press the ▼ key to scroll between 1, 3, 4, 5, 6, 9, or 12. Stop scrolling when the desired rate is dis-played. See Table 1 for the cycle rate options and the corresponding system equipment.
Press the ▲ key to display heating/cooling temperature control default.
Press the ▼ key again to change heating/cooling temperature control to C1 or C3.
- C1 = Standard heating/cooling temperature control.
- C3 = Aggressive heating/cooling temperature control (can cause room temperature to overshoot the setpoint).
Press the ▲ key again. Factory information (varies by model) is shown.

Table 1. Heating Cycle Rates.

System	Cycles Per Hour
Steam, gravity	1
Hydronic heat, condensing gas furnace	3
Gas or oil-forced air	6
Electric heat	9
Special applications	4, 5, 12

a high efficiency furnace.
b Refer to the equipment manufacturer’s instructions.

Press the ▲ key to save all changes, exit installer setup mode and return to normal operation.

NOTE: After exiting installer setup mode, slide the System switch to the desired position and change the setpoint to the desired room temperature.

CHECKOUT

Heating

Slide the SYSTEM switch to Heat and the FAN switch to Auto.
Press and hold the ▲ key to raise the temperature setting several degrees above the room temperature. After approximately ten seconds, the heating equipment should start. In conventional systems, the system turns on the fan through the use of a time delay relay or through a limit control. When using an electric heat thermostat, the fan starts immediately.
Press the ▼ key to lower the temperature setting below the room temperature. Heating equipment should stop.

Cooling

CAUTION

Low Temperature Hazard.

Operating at too low of an outdoor temperature may cause compressor damage. Do not operate cooling if the outdoor temperature is below 50°F (10°C). Refer to the manufacturer’s recommendations.

For compressor protection, during power interruption and on initial startup, the T8400 and T8401 Thermostats go into a five-minute delay. During this delay, the cooling icon (a snowflake ❄) flashes.

Slide the SYSTEM switch to Cool and the FAN switch to Auto.
Press the ▼ key to lower the temperature setting several degrees below the room temperature. After approximately five minutes, the cooling equipment should start. The fan starts and stops with the cooling equipment.
Press the ▲ key to raise the temperature above the room temperature. The cooling system should shut down.

NOTE: To bypass the five-minute delay, see the Optional System Checkout section.

Fan

Slide the SYSTEM switch to Off and the FAN switch to On. The fan should run continuously.
Slide the FAN switch to Auto. In heating, the fan is controlled directly by the heating equipment and may start a few minutes after the heating equip-ment turns on (on most systems). When using an electric heat thermostat, the fan starts and stops with the heating equipment. In cooling, the fan starts and stops with the cooling equipment.

Be sure all equipment responds correctly to the thermostat.

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